The present invention relates to a reference voltage source to be implemented as a monolithic integrated circuit using CMOS-transistor technology.
Reference sources of this general type have already been proposed, for example in the article of Yannis P. Tsividis and Richard W. Ulmer "A CMOS Reference Voltage Source", published in "1978 IEEE International Solid-State Circuits conference, Digest of Technical Papers", Vol. XXI, pages 48 and 49. According to this proposal a pair of MOS transistors having their gates connected to their drains and operating in the weak inversion region are used to derive from the difference between the voltages across these transistors a voltage having a positive temperature coefficient. This can be added to the voltage of a diode-connected bipolar transistor which has a negative temperature coefficient. This solution however comes out to combine two voltages which are strongly dependent on temperature and it is therefore very difficult, in practice, to obtain a resulting temperature independent voltage, in particular due to the dispersion of parameters in manufacture.
Another reference voltage source has been described by Robert A. Blauschild at al in the above mentioned review, pages 50 and 51, under the title "An NMOS Voltage Reference". This proposal uses the difference between the gate voltages of enhancement and depletion MOS transistors. It allows to obtain a reference voltage stable with temperature but the value of which depends on an implantation process. This process also requires an additional processing step which is applied selectively to one of the transistors and therefore increases the cost of manufacturing complementary silicon-gate transistors in which this step is normally not used.
A reference voltage source is also known (U.S. Pat. No. 3,975,648) which includes two substantially identical insulated-gate field-effect transistors having different flat-band voltage characteristics. These transistors are biased to carry equal drain currents at equal drain voltages and the resulting difference in potential between the gate contacts produces a slightly temperature-dependent reference voltage. However, the implementation of this known reference source is based on the use of different metals or of different semi-conductor materials to form the gates of the transistors. This leads to additional and costly manufacturing steps.
It is further already known that the threshold voltage required to invert a MOS device can be shifted and selectively controlled by using appropriately doped semiconductor material as the gate electrode (U.S. Pat. No. 3,673,471, published German patent application No. DE-OS 2,338,239).